Already known from U.S. Pat. No. 4,799,596 to Mallant (which has a German counterpart in DE 36 14 400) is a color sorting machine in which the product to be sorted is transported through a central duct. The duct is transparent in the area of an observation head. The observation head has, distributed about the duct, several light transmitters in the form of lamps which radiate through the transparent duct. A photocell or photodiode arrangement is arranged next to every lamp as product signal receiver. A background plate lies diametrically opposite the photocell arrangement at the other side of the duct, which plate is chosen according to the color of the product to be sorted. In operation, one transmitter always illuminates the front of the product and a second transmitter the background plate. The product signal and the background signal are recorded by the photocell arrangement and compared with each other. If the product signal corresponds to the background signal, the product has satisfied requirements. However, if the signals differ from each other, an ejector is triggered which discharges the corresponding product out of the product stream.
A disadvantage of the known device and of the process used is that the background plate always has to be physically matched to the product color, so that a large number of background plates with different colors always has to be kept in stock for every product. Moreover, the replacement of the background plates not only takes time but also calls for experience in the choice of the correct color.
It is known from U.S. Pat. No. 4,863,041 to change the background color dynamically for a comparable color sorting machine, by choosing for the background a light source whose wavelength can be changed by an electronic dimmer circuit. Although this relieves the user of the need to replace background plates physically, there remains the problem of sorting out items which do not completely cover the light beam sent out by the transmitter. As a result, the signal yield is only very low and small color deviations can be recognized only with difficulty.
It is known from DE 34 06 599 C2 to conduct product past light transmitters and light receivers so that light is reflected to the light receivers both from the front and from the rear of the product. The light receiver then sees the light of a background transmitter whenever no product is there. Although the transmitters lying next to the light receiver also radiate light, this light is not recorded by the light receiver because of the lack of a reflection area. If good product enters the ray path from the back- ground transmitter to the light receiver, the background radiation is wholly or partly interrupted, but the radiation from the transmitters is reflected onto the light receiver on a wavelength which corresponds to the background radiation. In the case of good product, the message to the light receiver is no different to what it would be were no product present at all. No distinction can thus be made between the presence or absence of good product.
Distinctions are only drawn when bad product crosses the ray path. It is thus not possible to count good product should this be desired for further process control. Nor can any good product be sorted out, which may be desirable for example if, after one or more sorting runs in which good product was separated from bad, inverse sorting is to be carried out, in which only sorted out material is once again fed through the sorting machine in order to sort out the good residual product which is inevitably still contained in it.
Known, finally, from DE 32 03 773 A1 is the use of a ground-glass plate as diffusor in conjunction with an optical color recognition system, for the recognition of the surface of a disc wheel. However, this has nothing to do with product sorting.
Therefore, one object of the invention is to provide an optical sorting system for, say, agricultural products such as coffee beans, peanuts etc. so that color deviations can be reliably recognized even if the product only partly covers the light beam from the transmitter, so that only part of the light quantity sent out is scattered back.